Internal combustion engines may include water injection systems that inject water into a plurality of locations, such as into an intake manifold, upstream of engine cylinders, or directly into engine cylinders. Engine water injection provides various benefits such as an increase fuel economy and engine performance, as well as a decrease in engine emissions. In particular, when water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to evaporate the water, leading to charge cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, improved wide-open throttle performance, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque. Furthermore, lowered combustion temperature with water injection may reduce NOx emissions, while a more efficient fuel mixture (less enrichment) may reduce carbon monoxide and hydrocarbon emissions.
Engine control systems may select when to use water injection based on engine operating conditions, such as engine knock limitations. One example approach is shown by Surnilla et al. in U.S. Pat. No. 8,096,283. Therein, water usage is based on water availability, knock limits, dilution requirements, and spark constraints. Another example approach is shown by Connor in U.S. Pat. No. 5,148,776. Therein water usage is adjusted based on the amount of cooling required to overcome premature ignition of an air-fuel mixture engine cylinders.
However the inventors herein have recognized potential issues with such approaches. As one example, the optimal fuel economy gain associated with water usage may not be realized due to the fixed gear ratio of the transmission. In particular, at a given driver demand, based on whether water is being injected or not, there may be an associated fixed engine speed and load range that meets the driver demand. An engine controller may use water injection based on water availability on-board the vehicle. However, when transitioning between operating with or without water injection, there may be engine limitations experienced at the associated engine speed-load that may reduce the fuel economy benefit of the transition. As an example, when water injection is not being used, the engine may become more knock-limited at high loads. Consequently, the optimum engine speed-load for the driver demand may be different from that when water injection is used. Another issue is that frequent changes in operator pedal demand may cause the engine load to move back and forth, leading to frequent switching on and off of water injection. Excessive switches can degrade fuel economy due to losses incurred during transitions, and may degrade the life of the parts, and may cause air-fuel disturbances that move away from ideal stoichiometry.
The inventors herein have recognized that the fuel economy benefits of an engine configured with water injection may be better leveraged through integration with a continuously variable transmission (CVT). In particular, the CVT may enable the engine speed and load to be adjusted based on water usage (and availability) while maintaining the power output of the engine. In one example, fuel economy may be improved by a method for an engine configured with water injection, the engine coupled with a continuously variable transmission (CVT), the method comprising: for a driver demanded power level, comparing fuel economy without water injection to fuel economy with water injection at a first adjusted engine speed-load; and in response to a higher than threshold improvement in the fuel economy with water injection at the adjusted engine speed-load, injecting an amount of water into the engine and changing to the first adjusted engine speed-load via the continuously variable transmission (CVT). In this way, an engine can be operated with water injection while providing an improved fuel economy for a given driver demand by increasing the maximum load that can be achieved without knocking, or in other words, by increasing the knock limit.
As one example, an engine may be configured with a water injection system that enables water to be injected into an intake manifold, into an intake port, or directly into an engine cylinder. Based on water availability (such as in a dedicated water tank), the water injection system may be in an active state (with water injection enabled) or an inactive state (with water injection disabled). At any given driver demand, the controller may be configured to compare the fuel efficiency versus power for each water injection state. The effects of knock limits associated with each water injection state are included in an efficiency versus power data included in the controller memory. If the current water injection state is not the more efficient state, the controller may predict if there are any limitations, such as knock limitations, associated with the corresponding engine speed-load. If so, the controller may further determine if the engine speed-load can be changed while maintaining the current water injection state and while maintaining the demanded engine power output, and any fuel penalties associated therewith. In other words, the controller may determine whether the optimum engine speed-load with the more cost efficient water injection state is different from the current engine speed-load. As the driver demand changes, if the efficiency of the current water injection state drops (e.g., by more than a threshold amount) below the efficiency of the other water injection state, the water injection state is switched. Else, the current water injection state is maintained. In particular, if the engine speed-load can be changed while maintaining usage of the current water injection state with a net fuel economy improvement, the controller may maintain the current water injection state and shift to the optimum speed-load range for the selected state. Else, the engine may switch to the other water injection state and shift to the optimum speed-load range for that state. As an example, when operating with water injection inactive, the engine may be knock limited at lower loads than when water injection is active. Therefore, to address knock anticipated while operating without water injection, an engine controller may need to actuate the CVT to increase the engine speed while decreasing the engine load so as to maintain the demanded engine power output, which may or may not provide a net cost benefit. Likewise, when operating with water injection active, the CVT may be actuated to lower the engine speed (relative to the previous engine speed when water injection was inactive) while load is increased (as compared to the previous load when water injection was inactive) which may provide a net cost benefit. Because the quantity of water is limited in the reservoir, the controller aims to only use the water when a pre-determined improvement in fuel efficiency will occur, so it only injects the water and adjusts the speed-load, when the “water” efficiency improvement exceeds a threshold over the non-water speed-load efficiency.
In this way, fuel economy benefits can be improved. The technical effect of integrating water injection technology in a vehicle having a CVT transmission is that for a given driver demanded power, the benefits of the water injection can be better leveraged. In particular, the engine speed and torque for a given driver demanded power can be adjusted to reduce knock limitations at higher loads to increase the maximum load, and reduce friction losses at lower loads, while accounting for changes in knock limits due to water injection properties. The technical effect of assessing the fuel economy benefit of switching water injection states with the fuel penalty associated with operating at the engine speed-load profile corresponding to a selected water injection state is that frequent switching between water injection states can be reduced. While operating the engine with the more efficient and cost-effective water injection state, CVT adjustments can be used to extend engine operation with water injection despite changes in driver or wheel torque demand, and for conditions where the benefit of water injection is small, CVT adjustments can be used to extend engine operation without water injection despite changes in driver or wheel torque demand. By optimizing water usage, the benefits of water injection can be extended over a longer portion of a drive cycle, even when water availability is limited.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.